1. Field
An aspect of the present invention relates to a secondary battery, and more particularly, to a secondary battery capable of being protected against external impact by reinforcing the strength of a pouch case constituting an outer casing of the secondary battery.
2. Description of the Related Technology
Secondary batteries are of great importance for they are used as power sources of many portable wireless devices such as a video camera, a mobile phone, a notebook computer and a portable personal digital assistant (PDA). Furthermore, since such secondary batteries can be used many times by recharging the batteries, they are more economical than disposable batteries.
A secondary battery is generally formed in the shape of a metallic can. A cylindrical or rectangular parallelepiped can has an opening formed at one side thereof, and an electrode assembly and an electrolyte are injected into the can through the opening. A bare cell is formed by sealing the can with a cap assembly having a size and shape corresponding to the opening.
Meanwhile, a nonaqueous electrolyte is used for a lithium secondary battery because of the reactive property of lithium and water. The electrolyte may be a solid polymer electrolyte containing lithium salt or a liquid electrolyte obtained by dissociating lithium salt in an organic solvent. Alternatively, the electrolyte may be an electrolyte obtained by infiltrating a liquid electrolyte into a separator. When the solid polymer electrolyte simultaneously serves as the separator and the electrolyte, the risk of liquid leaking is low, and therefore, a pouch may be used as a case for the electrode assembly and the electrolyte.
Pouch cases have light weight, small volume and adaptability in the shape of batteries or the installation method in devices as compared with cans made of metal. Therefore, it is advantageous to use the pouch cases for secondary batteries.
In a pouch type secondary battery, an electrical path through which an electrode in the interior of a pouch case is communicated with the exterior is secured using a tab connected to the electrode while sealing the pouch case having an electrode assembly accommodated therein. A core pack is formed by connecting a component or structure such as a protecting circuit module (PCM) or positive temperature coefficient (PTC) to a bare cell having the sealed pouch case using the tab. Then, a hard pack battery may be completed by inserting the core pack into a hard case and joining them together. Alternatively, an inner pack battery may be completed by adding a frame to the exterior of the core pack and surrounding them with a label for packaging.
Such a pouch case includes an accommodating portion having a groove in which an electrode assembly is accommodated, and a cover for covering the groove. Here, the groove is formed through press working, and the groove may not be formed based on the shape of the pouch case.
A general electrode assembly is formed into a multi-layered film in which a positive electrode, a separator and a negative electrode are sequentially stacked, or is formed into a jelly roll obtained by winding the multi-layered film. When the jelly roll is wound, a separator is additionally attached to the electrode surface exposed to the exterior of the jelly roll so as to prevent a short circuit between the positive and negative electrodes. When the jelly roll is put in a lower groove of the pouch case, the pouch case is sealed by tightly attaching front flanges to each other and then heating and pressurizing an edge portion protruded outward from the groove along the flanges.
Since the pouch case with such a structure is formed very thin, there is a limitation in protecting the electrode assembly accommodated in the pouch case using only the pouch case, and therefore, the pouch case is weak to external impact.
Further, the electrode assembly accommodated in the pouch may cause a swelling phenomenon where gas is generated in the interior of a battery as the battery is initially charged or discharged. That is, when gas is generated in the interior of electrode plates, the electrode plates are swelled in both side directions of the pouch case, and the volume of the pouch case is expanded. Therefore, a short circuit is formed at an edge portion of the positive and negative electrode plates.